Parthood and Composition in Quantum Mechanics

The paper deals with parts and wholes in quantum theory. It addresses a much neglected question, namely of what mereological theory are quantum systems a model of. It argues that they are at least a model of the so called Closed Extensional Mereology. It then goes on to address the question of whether quantum theory favors a particular answer to what is known as the special composition question, i.e. what are the sufficient and necessary conditions a set of entities has to meet in order to have a mereological sum. It is suggested that quantum mechanics by itself falls short to yield a definitive answer to that question and different possible suggestions are explored. One of them is that quantum theory, together with some mild assumptions, such as the one that maintains that there are no uninstantiatedproperties, delivers at least a sufficient condition for having a mereological sum. This condition turns out to be that the quantum system is in an entangled state.